Schema migration tools.

[amcgregor]

From #python:

<ssbr_at_work> IDK. These days when I look at schemas, the core question I want to have answered is, "how do I keep forward and backward compatibility?"
<ssbr_at_work> File formats need to answer that question, universally, don't they? Why don't the schema formats help?
<ssbr_at_work> GothAlice: IMO you should borrow extensively from cap'n proto / protocol buffers

<ssbr_at_work> GothAlice: So you write a serialization protocol that has a lot of freedom for forward/backward compatibility reasons, and then for a specific version you write a validator that ignores things it doesn't know about.
<ssbr_at_work> GothAlice: so IMO the only thing marrow should remember from the lessons learned by protobuf/cap'n proto is not to reject unknown values
<sproingie> ssbr_at_work: i think it's that protocol compatibility is much more complicated than locking it down to a particular version number or required fields, so it punts and leaves it up to the application layer to decide unless you avail yourself of explicitly marking things required
<ssbr_at_work> sproingie: marking things as required is an antipattern in protocol buffer land, to the extent that it's forbidden in cap'n proto
<ssbr_at_work> GothAlice: protocol buffers have a few ways they deal with unknown values that might be interesting to you as alternate options (or defaults)
<ssbr_at_work> GothAlice: in particular, unknown fields are totally ignored, unknown values for known fields are read as a "default" known value